Origin Of Heartbeat/ECG/Cardiac APs

Question 1

What is the difference between fast and slow cardiac action potentials in terms of where they are found?

Answer 1

Fast cardiac APs are in atria, ventricles, and conduction system

Slow cardiac APs are in SA and AV nodal tissues

Question 2

Fast cardiac APs are very rapidly conducting but non-contractile in ________________ fibers

They are rapidly conducting AND contractile in ______ and _______ fibers

Answer 2

Purkinje

Atrial; ventricular

Question 3

What is the difference in amplitude between fast and slow cardiac APs?

Answer 3

Fast cardiac APs are high amplitude (100 mV)

Slow cardiac APs are low amplitude

Question 4

What is the difference in ion conductance between fast and slow cardiac APs?

Answer 4

Fast cardiac APs are due to changes in conductance of K, Na, and Ca

Depolarization in slow cardiac APs is due to calcium; there are no fast sodium gates

Question 5

What is the difference in resting phase potential in fast vs. slow cardiac APs?

Answer 5

Fast cardiac APs = -90

Slow cardiac APs = -60

Question 6

What is the pre-potential?

Answer 6

Tendency of the AP of cardiac cell membrane to drift towards threshold following repolarization

Question 7

What is the conducting pathway through the heart starting with the SA node?

Answer 7

SA node –> atrial muscle –> internodal pathway –> AV node –> bundle of His –> bundle branches –> purkinje system –> ventricular myocytes

Question 8

When the vector in the heart is in a direction almost perpendicular to the axis of the lead, the voltage recorded in the ECG of the lead is very ______

Answer 8

Low

Question 9

What happens on the ECG when the heart vector has almost the same axis as the axis of the lead?

Answer 9

The entire voltage of the vector will be recorded

Question 10

Which wave on the ECG corresponds to phase 0 of APs spread through the ventricles?

Answer 10

QRS

Question 11

Which wave on the ECG corresponds to phase 0 of APs spread through atrial muscle?

Answer 11

P wave

Question 12

The T wave on the ECG represents which phase of cardiac muscle contraction?

Answer 12

Phase 3 = repolarization of ventricular muscle fibers spreading through the ventricles

Question 13

Which part of the ECG corresponds to the nodal delay during cardiac contraction?

Answer 13

PR interval

Question 14

Which part of the ECG is corrected for heart rate?

Answer 14

QT interval (becomes QTc)

Question 15

Which part of the ECG corresponds to the amount of time it takes for contraction to go from atria to ventricles?

Answer 15

PR interval

Question 16

Which part of the ECG corresponds to the total time ventricles spend depolarized?

Answer 16

QT interval

Question 17

Compare conduction time through the AV node vs. His-Purkinje system

Answer 17

Conduction through AV is slow to allow ventricles time to fill prior to contraction

Conduction through the His-Purkinje system is fast as it rapidly distributes the AP to ventricles

Question 18

What are the phases of ventricular action potential?

Answer 18

Phase 0 = upstroke aka depolarization

Phase 1 = initial repolarization (gradual)

Phase 2 = plateau

Phase 3 = repolarization (rapid)

Phase 4 = resting potential

Question 19

What causes phase 0 of ventricular action potential

Answer 19

Transient increase in Na conductance produced by depolarization induced activation gates on sodium channels

Question 20

What is occuring during phase 1 of the ventricular action potential

Answer 20

Gradual, initial repolarization

Net outward current due to closure of sodium inactivation gates and large electrochemical forces favoring outflow of K+

Question 21

What causes the plateau during phase 2 of cardiac action potential?

Answer 21

Inward current of Ca = outward current of K

Question 22

The increase in calcium conduction during phase 2 of the ventricular action potential causes a slow inward Ca current via ________ channes

Answer 22

L-type

Question 23

Describe changes in ion conductance during phase 3 of the cardiac action potential

Answer 23

Decrease in inward Ca current

Increase in outward K current

Question 24

Cardiac action potentials vary in duration. In which of the following would they be the longest?

A. Purkinje fibers
B. Ventricles
C. Atria
D. Skeletal muscle

Answer 24

A. Purkinje fibers

Question 25

The longer the action potential, the ______ the refractory period

Answer 25

Longer

Question 26

What accounts for the length of the cardiac action potential?

Answer 26

Plateau, which is a sustained period of depolarization in the atria, ventircles, and purkinje fibers

Question 27

________ effects change the rate of depolarization of SA node and therefore heart rate

Answer 27

Chronotropic

Question 28

________ effects influence the speed of conduction

Answer 28

Dromotropic

Question 29

T/F: conduction velocity through the AV node is partially dependent upon action potential duration

Answer 29

FALSE

Question 30

What are some factors that influence conduction velocity through the AV node?

Answer 30

Larger fibers have faster transmission of AP Parasympathetic effects decrease conduction velocity through AV node Sympathetic activity increases conduction velocity through AV node

Question 31

What phases are absent from the AP in the SA node?

Answer 31

1 and 2

Question 32

What are the phases of the AP in the SA node?

Answer 32

Phase 0 = upstroke Phase 3 = repolarization (net outward K+ current) Phase 4 = spontaneous depolarization or pacemaker potential

Question 33

What is the longest portion of the SA AP and thus is responsible for setting the heart rate?

Answer 33

Phase 4 = spontaneous depolarization or pacemeker potential

Question 34

What are the latent pacemakers of the heart and which one has the fastest intrinsic rate of activity?

Answer 34

``` AV node (fastest at 40-60 bpm) Bundle of His (40 bpm) Purkinje fibers (15-20 bpm) ```

Question 35

What causes PVCs on the ECG? PACs?

Answer 35

PVCs can be from development of new pacemaker foci in ventricles PACs can be from development of new pacemaker foci in atrium away form the SA node